One of the limitations of using poly(vinylidene fluoride), acronymn PVDF, resins as a material of construction in forming, for example, molded parts for pumps and valves, is the decrease in load bearing strength which occurs when the resins are heated to elevated temperatures. Another limitation of poly(vinylidene fluoride) is that it shrinks from 2 to 3% on cooling a hot molding of the resin. By incorporating carbon fibers into the resin, for example, as is described in British Pat. No. 1,324,424, its strength is improved and shrinkage on molding is reduced.
Compressed, expanded vermicular graphite, which has been coated with corrosion resistant resins including PVDF, has been disclosed as a coating material and laminate in U.S. Pat. Nos. 3,438,932 and 4,199,628. Articles formed from reinforced plastics, containing carbon or graphite fibers coated with epoxy polymers, are disclosed in U.S. Pat. No. 4,107,128. Other patents relating to carbon reinforced compositions include: U.S. Pat. No. 4,009,043 which discloses molding compositions of polymers, which can be halogen containing, along with both titanium and carbon fibers; and U.S. Pat. No. 3,885,174, in which carbon fibers coated with certain block or graft copolymers, including polyesters, epoxy, and polyimide resins are mixed with a compatible resin, i.e., one component of the graft copolymer. In the background section of U.S. Pat. No. 3,964,952, it is mentioned that, where carbon fibers are impregnated with a resin and then immersed in the same resin to form a composite, the composite has poor mechanical properties. In U.S. Pat. No. 3,682,595, a non-woven carbonaceous fabric is prepared by coating carbonaceous fibers with a resin such PVDF and then carbonizing the system.
Carbon fibers which have been coated with polyvinylpyrrolidone or an epoxy resin in order to improve their bulk density and handling characteristics, are commercially available. These coatings, however, are sensitive to hot acids and I have found that composites of PVDF and such fibers have significantly reduced tensile strength when they are immersed in hot hydrochloric acid, even though the PVDF matrix resin is acid resistant, because of wicking. This makes the composites unsuitable for making valves, pumps and other parts which will be exposed to acids. I have now prepared poly(vinylidene fluoride)-carbon fiber reinforced composites having increased tensile strength, lower mold shrinkage and increased stability to hot aqueous acids.